38 Electrocatalysis and the splitting of water
2.5.3 Catalyst stability
In the previous sections it has been briey mentioned that the OER catalysts
have to survive in extremely harsh and oxidising conditions. It is therefore of
utmost importance that new and active catalysts are actually stable in those
conditions for an extended amount of time. However, while searching for new
materials long term testing is not often a focus point and instead many groups
turn to shorter electrochemical tests (2-24 hours) at either constant potential
or constant current density. The question is whether these tests reveal any
stability that can be extrapolated to the long term. A pure electrochemical test
at constant current density have traditionally been the standard 109. This type
of test is justied by the operation mode such a catalyst would typically work
under in an actual device. A certain amount of hydrogen is needed or a certain
amount of energy is used and the electrolyzer is therefore likely to operate under
constant current density. If the catalyst deactivates over time it leads to an
increase in the applied potential in order to reach the constant current density.
However, since the current is exponentially dependent on the overpotential small
changes in potential lead to huge changes in current. So when measuring the
potential at constant current the changes are normally within 50 mV, which
may be judged acceptable from the researchers point of view. This type of
test does not inform the researcher about any specic changes in the catalyst
structure, purity or electrode thickness. More importantly it does not reveal
when a complete loss of the catalyst material will occur. Such information must
be analysed with other means which is the topic of this section.
Recently, stability tests have received increased attention in the eld of OER,
likely inspired by the importance that stability plays in the evaluation of ORR
catalysts 169174. For Pt based ORR catalysts a typical accelerated stability
test consists of 10000 cycles in a relevant potential range after which the loss of
activity is reported 175177. The ORR proceeds at potentials where Pt is thermodynamically
stable but the start-stop conditions in fuel cells can cause surface
oxidation followed by reduction, resulting in loss of platinum 173. Conversely,
for OER catalysts many materials are prepared in a state that is not thermodynamically
stable at potentials higher than 1.6 VRHE. For prolonged operation
at high current densities the surface is bound to change structure and this can
lead to a loss of catalyst material. Such eects have been elegantly shown for
a selection of noble metal oxides by Cherevko et al. 178180. In those studies
an online Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) system
is used to measure corrosion products while the catalysts are under potential
control. A short introduction to ICP-MS can be found in section 3.4.2. The
results can be seen in gure 2.13 for Ru, Ir, Pt and Au, where dissolution rates
and voltammetric proles are shown. The potential is slowly scanned between
0 and 1.5 VRHE, while the dissolved species are measured with the ICP-MS.
Notice that for Ru the dissolution occurs solely at the peak potential whereas
it is stable at all other potentials. This is in contrast to the other three metals,